The present invention relates to a ferroelectric film, a ferroelectric capacitor, a ferroelectric memory, a piezoelectric element, a semiconductor element, a method of manufacturing a ferroelectric film, and a method of manufacturing a ferroelectric capacitor.
It has recently become popular to perform research and development into ferroelectric films of PZT or SBT or the like, as well as devices such as ferroelectric capacitors and ferroelectric memory devices that use such films. The configurations of ferroelectric memory devices are categorized into 1T type, 1T1C type, 2T2C type, and simple matrix type. Of these, the structure of the 1T type leads to the generation of internal electrical fields which shorten the retention (data preservation) to one month, so it is thought to be impossible to provide a guarantee of ten years, which is generally requested of semiconductors. The 1T1C type and the 2T2C type have mostly the same configuration as DRAM and have selection transistors, so DRAM fabrication techniques can be used therefor. Since the 1T1C type and the 2T2C type implement write speeds similar to those of SRAM, they are currently being used in small-capacitance capacitance products of 256 Kbit or less.
The ferroelectric materials used up until now have mainly been Pb(Zr, Ti)O3 (PZT). With PZT, the ratio of Zr to Ti is 52/48 or 40/60 and the composition that is used is a region that is a mixture of trigonal and tetragonal crystals, or the vicinity thereof. With PZT, materials that have also been doped with elements such as Lz, Sr, or Ca are used. These regions are used because they guarantee the reliability that is most necessary for a memory element. A tetragonal region that is rich in Ti has a favorable hysteresis shape, but Schottky defects caused by the ionic crystal structure occur therein. For that reason, failures occur in the leakage current characteristic or imprint characteristic (measures of hysteresis distortion), making it difficult to ensure reliability.
A simple matrix type of memory cell, on the other hand, has a cell size smaller than those of the 1T1C and 2T2C types, and it is also possible to stack capacitors, so it is promising for high integration, inexpensive applications.
Details of a conventional simple matrix type of ferroelectric memory device are given in Japanese Patent Laid-Open No. 9-116107. This publication discloses a drive method by which a voltage that is ⅓ of the write voltage is applied to non-selected memory cells when data is written to the memory cells.
However, details concerning the hysteresis loop of the ferroelectric capacitor, which is necessary for operation, are not specifically disclosed therein. Good squareness of hysteresis loop is essential for obtaining a simple matrix type of ferroelectric memory device that can operate in practice. Ti-rich tetragonal PZT can be considered as a candidate for the ferroelectric material that can be applied thereto, but the guaranteeing of reliability is the most important technical concern therewith, in a similar manner to the 1T1C and 2T2C types of ferroelectric memory.
PZT tetragonal crystals exhibit a hysteresis characteristic that has the squareness suitable for memory applications, but they lack reliability and cannot be used in practice. The reasons for this are discussed below.
First of all, a PZT tetragonal thin film tends to have a high leakage current density after crystallization, which increases as the ratio of Ti contained therein increases. In addition, static imprint testing in which data is written once in either the positive or negative direction and the memory device is heated and held at 100° C. has shown that most of the written data disappears after 24 hours. These problems are intrinsic to the ionic crystals of PZT and to the Pb and Ti that are constituent elements of PZT, and create the greatest technical problem relating to PZT tetragonal thin film in which large proportions of the constituent elements are Pb and Ti. This technical problems is great because PZT Perovskite is ionic crystals, and is intrinsic to PZT.
A list of the main energies involved in the bonds between the constituent elements of PZT is shown in FIG. 44. It is known that PZT includes many oxygen vacancies after crystallization. In other words, it can be expected from FIG. 44 that Pb—O bonds have the smallest bond energy among the constituent elements of PZT and will simply break during baking or polarization inversions. In other words, if Pb escapes, O will also escape for reasons of charge neutrality.
During sustained heating such as imprint testing, the constituent elements of PZT vibrate and collide repeatedly, and the Ti that is the lightest constituent element of PZT can easily be knocked out by these vibrational collisions during high-temperature retention. Therefore, if Ti escapes, O will also escape for reasons of charge neutrality. Since the maximum valence of +2 for Pb and +4 for Ti contribute towards bonding, there is no way to maintain charge neutrality other than allowing O to escape. In other words, two negative O ions escape readily for every positive ion of Pb or Ti, so that Schottky defects easily form.
The description now turns to the mechanism of the generation of leakage currents due to oxygen lack in PZT crystals. FIGS. 45A to 45C illustrate the generation of leakage currents in oxide crystals having a Brownmillerite type of crystal structure described by the general formula ABO2.5. As shown in FIG. 45A, the Brownmillerite type of crystal structure is a crystal structure having an oxygen insufficiency in comparison with the Perovskite type of crystal structure of PZT crystals having the general formula ABO3. As shown in FIG. 45B, since oxygen ions appear in the vicinity of positive ions in the Brownmillerite type of crystal structure, positive ion defects make it difficult for excessive leakage current to increase. However, oxygen ions link the entire PZT crystal in series as shown in FIG. 45C, and leakage currents increase accordingly in the case of a Brownmillerite type of crystal structure, in which the oxygen vacancy is larger than the above description.
In addition to the above-described generation of leakage currents, insufficiencies of Pb and Ti and the concomitant insufficiency of O, which are lattice defects, cause spatial charge polarization such as that shown in FIG. 46. When that happens, reverse electrical fields due to lattice defects are created by the electrical fields of ferroelectric polarization can occur, causing a state in which the bias potential is impeded in the PZT crystals, and hysteresis shift or collapse can occur as a result. Furthermore, these phenomena are likely to occur quicker as the temperature increases.
The above problems are intrinsic to PZT and it is considered difficult to analyze these problems in pure PZT, so that up until now it has not been possible to implement suitable characteristics for a memory element made by using tetragonal PZT.
In ferroelectric memory, one factor that determines the characteristics of the device is the crystallization state of the ferroelectric film included within the ferroelectric capacitor. The process of manufacturing ferroelectric memory has processes for forming an interlayer dielectric and a protective film, and processes that generate large quantities of hydrogen are used. Since the ferroelectric film at this point is mainly formed of oxides, the oxides are reduced by the generated hydrogen during the fabrication process, which has an undesirable effect on the characteristics of the ferroelectric capacitor.
For that reason, a resistance to reduction is secured for the ferroelectric capacitor in the conventional ferroelectric memory by covering the capacitor with a barrier film such as an aluminum oxide layer or an aluminum nitride layer, to prevent deterioration of the characteristics of the ferroelectric capacitor. However, such a barrier film necessitates the use of extra real estate during the integration of the ferroelectric memory, making it desirable to find a method that enables the manufacture of ferroelectric memory by a simpler process, from the productivity point of view as well.